The present invention relates to a plant for treating a rolled steel product, such as concrete reinforcing rod or bar (referred to simply as "rod" below), in order to improve its quality.
The main qualities required by users of steel rods are, among other things, as high as possible a yield strength for the kind of steel used, as well as satisfactory weldability, fatigue strength, and ductility for the use to which the rod is to be put. On the other hand, in order to improve weldability and ductility of a steel, it is necessary to decrease its carbon and manganese content, which concurrently results in a decrease of its tensile strength. To remedy this drawback, the steel can undergo a suitable cooling treatment, preferably directly applied while the steel emerges from the rolling mill, which permits the yield strength of the rod to be raised to a certain extent.
When the rod (particularly concrete reinforcing rod) is cooled by convection or radiation, the way in which the rolled product cools depends almost entirely on the diameter of the rod so that, for a rod of a given diameter, it is necessary to make use of other procedures for completing the mere cooling action in order to modify its yield strength, which results in an increase of the cost of the product.
To avoid the above-mentioned drawback, without increasing the carbon and manganese content of the steel up to an unacceptable value from the weldability viewpoint, the applicants have already suggested a process for treating rods in which a rod undergoes surface quenching at the exit of the finishing stand of a rolling mill by means of a suitable cooling fluid; the conditions of cooling are adjusted so that, at the outlet of the quenching zone, the core of the rod remains at a temperature sufficient to allow, during subsequent air cooling, tempering of the surface quenched layer, and transformation of austenite into ferrite and carbides in the central part of the rod. Depending upon the conditions in which this process is performed, the surface layer, due to quenching of the rod, is formed of martensite or bainite.
In practice, the desired cooling of the rod is obtained by appropriately choosing the cooling apparatuses such as, for example, sprayers, and by suitably adjusting their extent and their position.
In order to be able to increase the efficiency of the cooling apparatus, we have found it important to be able to ensure that in the cooling zones the rod is in fact sprayed in the desired manner with the cooling liquid, and that between the cooling zones and after the last of these zones, the rod is prevented from contacting the liquid; otherwise the structure or structures of the rod are liable to follow an evolution which would no longer allow the desired properties to be obtained.